The invention relates to a control circuit for controlling a DC supply voltage for a terminal, particularly for an analogue telephone of a public telephone network.
The telephone connection represents a network access to a telephone network (PSTN: public switched telephone network). Apart from the subscriber terminal, the in-house cabling, the subscriber line (TAL) and the subscriber line circuit in the telephone exchange (TVSt), the telephone connection also comprises the capability of usage contractually agreed with the network operator.
Subscriber terminals (TE: Terminal Equipment) can be both telephones and fax machines, PCs with modem or other technical facilities suitable for use in the network. A telephone connection can be arranged to be both analogue and digital. An analogue telephone connection is also referred to as POTS (plain old telephone system).
As a rule, telephones are connected with twisted pairs of telephone wires (wire pair), the interface to the connection of analogue terminals being called an a/b interface. The wire pairs are also designated as tip/ring. The a/b interface can be defined by various interface parameters, for example the loop current, the loop interruption, the idle and busy state and call dialling and talk state. Furthermore, a number of conditions must be met, for example attenuation distortion or noise level. These parameter conditions are prescribed specifically for each country.
FIG. 1 shows a conventional arrangement. A telephone is connected via a two-wire telephone subscriber line to a so-called SLIC circuit (subscriber line interface circuit). The SLIC circuit handles the so-called BORSCHT functions, where BORSCHT is an artificial word which stands for the following functions:
Battery (power feed),
Overvoltage (overvoltage protection),
Ringing (switching the caller to the subscriber),
Signalling,
Coding (DA/AD conversion and PCM encoding)
Hybrid (two-wire/four-wire conversion) and
Test (error detection).
Signalling is also understood to mean the supervision, i.e. the monitoring of the off-hook/on-hook state of the telephone. The analogue voice signal is transmitted in a voice frequency band within the range of about 0.3 to 3.4 kHz.
The telephone connection also provides the subscriber with access to the Internet by means of a temporary dial-up connection (dial-in) with interaction of a service provider. To transmit data in analogue telephone networks, voice band modems are used. In the telephone network, these behave like a telephone, i.e. for the data transmission, the digital signals are converted into analogue signals in the frequency band between 0.3 to 3.4 kHz.
Among other things, the SLIC circuit shown in FIG. 1 contains a current sensor for sensing the loop current on the telephone subscriber line. The loop current to the telephone subscriber line comprises a direct-current component for supplying the telephone and an alternating-current component for transmitting information, particularly voice information. The current sensor contained in the SLIC circuit delivers the sensed current, scaled, via a line to a subsequent so-called CODEC circuit. The CODEC circuit handles the analogue/digital conversion and, if necessary, PCM coding of the signal.
As can be seen from FIG. 1, the line current sensed by the SLIC circuit leads to a voltage drop across a resistor RSENSE. The CODEC circuit contains an analogue connecting loop with a subtractor. A voltage generator generates a direct voltage of, for example, 1.2 V, the polarity of which can be inverted for signalling purposes. The voltage drop across the sensing resistor RSENSE, which is proportional to the sensed line current, is subtracted from the generated direct voltage by means of the subtractor for generating a difference voltage. Apart from scaling the sensed current, the SLIC circuit also performs current limiting so that current peaks are limited, for example, to 60 mA. As can be seen in FIG. 1, the difference voltage generated in the CODEC circuit is delivered to a low-pass filter which comprises a resistor and an external capacitor CF. The filtered difference voltage is applied to an input of the SLIC circuit which outputs the filtered difference voltage, amplified by an integrated amplifier, via stabilization and protective resistors to the telephone subscriber line of the telephone. The protective resistors have, for example, a resistance value of 50Ω and are used, for example, for protection against overvoltages which can be caused by a lightning stroke and for stabilization against oscillations.
FIG. 2 shows a conventional current/voltage characteristic of the voltage control shown in FIG. 1. The line current flowing on the telephone subscriber line TAL depends on the applied telephone line voltage. There is a predetermined open-loop voltage V0 which is, for example, 48 V. Apart from the resistance of the telephone, the load applied to the SLIC circuit comprises the resistance of the telephone subscriber line TAL and the resistance of the stabilization resistors:RLOAD=RTEL+RLINE+RSTAB.
In this context, the resistance of the telephone is dependent on whether the analogue telephone is on-hook or off-hook. In the on-hook state, the resistance value of the telephone is very high and is some MΩ. In the off-hook state, the resistance of the analogue telephone is much lower and is, for example, between 10 and 600Ω. Due to the high telephone resistance, the operating point with the telephone replaced in the on-hook mode is located at the bottom right on the characteristic shown in FIG. 2. If the telephone is removed, an operating point AP which depends on the load resistance RLOAD is obtained on the characteristic. The lower the load resistance, the further away is the operating point in off-hook mode from the operating point in the on-hook state. In FIG. 2, two operating points AP1, AP2 are drawn by way of example, the load resistance being 430Ω at operating point AP1 and 100Ω at operating point AP2. The lower the load resistance RLOAD, the higher the line supply current for the telephone.
The direct-voltage supply control according to the prior art, shown in FIG. 1, has the disadvantage that it is not suitable for long telephone lines. The longer the telephone line, the higher the load resistance RLOAD will be. The line current for supplying the telephone line must not drop below a certain minimum threshold value ILINE-MIN which is, for example, 18 mA. The longer the telephone line, the higher the load resistance RLOAD will be and the more the operating point AP in the off-hook state of the telephone will move towards the on-hook operating point and, at the same time, the supply line current ILINE will drop. If the supply line current drops below the predetermined threshold value, the telephone is no longer adequately supplied with energy.
A digital control circuit for controlling a DC supply voltage as shown in FIG. 3 has been proposed, therefore. In this arrangement, this digital control loop is integrated in a CODEC circuit.
FIG. 4 shows the entire CODEC circuit which contains a conventional digital control loop. The current sensed and scaled by the SLIC circuit is supplied to an analogue filter which separates it into an alternating-current component for the voice information contained in the current and into a direct-current component.
The alternating-current component is split off by means of a high-pass filter and, after filtering by an analogue prefilter contained in the CODEC circuit, is converted into a digital voice data signal by an AC analogue/digital converter. After downsampling by means of a downsampling unit, the digital voice data signal delivered by the terminal is again filtered by a digital AC filter and supplied to an adder which adds a digital signal delivered by a digital echo compensation filter for generating an aggregate signal which is evaluated via a further data processing unit, not shown.
The external resistance divider generates at a second input of the CODEC circuit a voltage proportional to the direct current flowing on the subscriber line, which is applied to the digital direct-voltage control loop contained in the CODEC circuit. An analogue low-pass prefilter contained in the CODEC circuit filters the applied voltage and applies this filtered voltage to the input of a DC analogue/digital converter which converts the applied voltage into a digital output signal. After downsampling by means of a downsampling unit, the direct-voltage value generated is delivered to a digital control circuit integrated in the CODEC circuit. This control circuit generates a voltage value, which is dependent on the direct current flowing on the subscriber line and, after upsampling by an upsampling unit, is supplied to a digital/analogue converter integrated in the CODEC circuit. The direct voltage generated by the digital/analogue converter is then applied, filtered by a low-pass filter, to a signal input of the SLIC circuit and, amplified there, is delivered to the terminal subscriber line.
Apart from the AC receive signal path for receiving the information data delivered by the terminal, the CODEC circuit also contains an AC transmit signal path for delivering information data to the terminal. For this purpose, a data signal coming from a data source is first filtered by a digital AC filter and delivered to an adder which adds a digital output signal from a digital impedance matching filter to form an aggregate signal which is upsampled by an upsampling unit and is then converted into an analogue signal by a further digital/analogue converter DAC. After filtering by an analogue output filter of the CODEC circuit, the alternating-voltage signal, amplified by a signal amplifier contained in the SLIC circuit, is also delivered to the subscriber line of the terminal.
The conventional direct-voltage control device for a terminal, shown in FIG. 4, has some disadvantages. Separation between the alternating-voltage signal sensed by the SLIC circuit and the direct-voltage signal is effected by an external analogue filter which, as can be seen from FIG. 4, comprises two resistors and two capacitors. These components cannot be integrated into the CODEC circuit according to the prior art. Accordingly, the CODEC circuit needs four terminal pins, namely two input terminal pins for the alternating-voltage signal and the direct-voltage signal and two output pins for the controlled direct-voltage output signal and the alternating-voltage signal for the terminal.
A further disadvantage of the CODEC circuit with integrated digital control loop, shown in FIG. 4, consists in that two separate analogue/digital converters ADC are needed, namely one for the received alternating-voltage signal (AC ADC) and a further analogue/digital converter for the received direct-voltage signal (DC ADC). In addition, the CODEC circuit shown in FIG. 4 needs two separate downsampling units. Providing two separate analogue/digital converters, in particular, considerably increases the required area in the case of integration on a chip.
It is an object of the present invention, therefore, to create a control device for controlling a supply voltage for a terminal which ensures an adequate power supply even for a terminal connected via a long subscriber line with minimum complexity in terms of circuitry.